Printing system with a negative working thermal plate for onpress development

Abstract

A printing system making use of a lithographic printing plate has been disclosed, the system comprising the steps of image-wise exposing to infrared light a heat sensitive imaging element, the element being optionally present on the printing press before starting the image-wise exposing step to infrared light, wherein the element comprises, on a lithographic base with a hydrophilic surface thereupon, an image-forming layer including hydrophobic thermoplastic polymer particles and a hydrophilic polymer binder, and, optionally, an infrared absorbing compound, wherein the hydrophobic polymer particles contain more than 0.1 wt % of nitrogen and have an average particle size diameter in the range from 0.015 to 0.150 mum; developing the image-wise exposed imaging element by mounting it on a print cylinder of a printing press and applying an aqueous dampening liquid ink to the imaging element while rotating the print cylinder; providing a printing run length of the press, increased with a factor of at least 5, when reducing the average particle size diameter of the hydrophobic polymer particles in an amount of more than 25%.

What is claimed is:  
     
       1. Method of printing on a press, providing a printing run length of said press to be increased with a factor of at least 5 versus a reference run length, making use therefore of a lithographic printing plate, said method comprising the steps of: 
       image-wise exposing to infrared light a heat sensitive imaging element, said element being optionally present on the press before starting said image-wise exposing step to infrared light, wherein said element comprises, on a lithographic base with a hydrophilic surface thereupon, an image-forming layer including hydrophobic thermoplastic polymer particles and a hydrophilic polymer binder, and, optionally, an infrared absorbing compound, wherein said hydrophobic polymer particles contain more than 0.1 wt % of nitrogen and have an average particle size diameter in the range from 0.015 to 0.15 μm, and wherein said reference run length utilizes reference particles containing no or less than 0.1 wt % of nitrogen,  
       developing the image-wise exposed imaging element by mounting it on a print cylinder of a printing press and applying an aqueous dampening liquid and ink to said imaging element while rotating said print cylinder, starting printing up to said increased run length.  
     
     
       2. Method according to  claim 1 , wherein said hydrophobic polymer particles contain structural chemical groups selected from the group consisting of amide, urethane, methacrylonitrile, crotonitrile, vinylidene cyanide, isocytosine, pyrrolidone, piperazine, cyanomethyl, cyanoethyl, cyanopropyl, cyanoaryl, cyanoacrylate, primary amines, mono- or di-n-alkyl substituted amines, urea, imide, imine, triazine, sulfonamide, onium, melamine, pyrimidine, ureido-pyrimidone, pyridine, barbiturate, isocyanurate or imidazole. 
     
     
       3. Method according to  claim 1 , wherein said hydrophilic polymer binder is a water-soluble, water-dispersable, alkali-dispersable or alkali-soluble polymer. 
     
     
       4. Method according to  claim 1 , wherein the hydrophobic thermoplastic polymer particles consist of a copolymer of monomers selected from the group consisting of styrene, tert.-butylstyrene, methylmethacrylate, peramethylstyrene, methacrylonitrile, N-alkly substituted acrylamides, N-alkyl substituted methacrylamides and maleimides. 
     
     
       5. Method according to  claim 1 , wherein the hydrophobic thermoplastic polymer particles are present in the image forming layer in an amount of at least 50 wt %. 
     
     
       6. Method according to  claim 1 , further comprising a second hydrophilic polymer binder in a layer adjacent to said image forming layer. 
     
     
       7. Method according to  claim 1 , wherein the infrared absorbing compound is an anionic infrared cyanine dye absorbing infrared radiation in the wavelength range from 800 to 1100 nm and wherein the infrared absorbing compound is present in said image forming layer or in a layer adjacent thereto. 
     
     
       8. Method according to  claim 1 , wherein the hydrophilic surface is a lithographic surface, present on a metal support, being a plate or a print cylinder. 
     
     
       9. Method of printing on a press, said method comprising the steps of: 
       image-wise exposing to infrared light a heat sensitive lithographic printing plate, wherein said lithographic printing plate comprises:  
       a lithographic base with a hydrophilic surface thereupon, an image-forming layer including hydrophobic thermoplastic polymer particles and a hydrophilic polymer binder, and, an infrared absorbing compound, wherein said hydrophobic polymer particles contain more than 0.1 wt % of nitrogen and have an average particle size diameter in the range from 0.015 to 0.15 μm,  
       developing the image-wise exposed printing plate by mounting it on a print cylinder of a printing press and applying an aqueous dampening liquid and ink to said imaging element while rotating said print cylinder, and  
       printing.  
     
     
       10. The method of  claim 9  wherein said lithographic printing plate is present on said press prior to said image-wise exposing. 
     
     
       11. Method of printing on a press, said method comprising the steps of: 
       image-wise exposing to infrared light a heat sensitive lithographic printing plate, wherein said lithographic printing plate comprises:  
       a lithographic base with a hydrophilic surface thereupon, an image-forming layer including hydrophobic thermoplastic polymer particles and a hydrophilic polymer binder, and, an infrared absorbing compound, wherein said hydrophobic polymer particles contain more than 0.1 wt % of nitrogen and have an average particle size diameter in the range from 0.015 to 0.15 μm,  
       developing the image-wise exposed imaging element by mounting it on a print cylinder of a printing press and applying an aqueous dampening liquid and ink to said imaging element while rotating said print cylinder, and  
       printing;  
       with the proviso that said printing is at least 5 times longer than printing when said hydrophobic polymer particles contain less than 0.1 wt % nitrogen and have an average particle diameter more than 0.15 μm wherein said lithographic printing plate is present on said press prior to said image-wise exposing.